It was in a cab driving down Fifth Avenue, passing an apartment building on E. 66th Street, that Michael Gross, author of a book about the rich and famous residents of 740 Park Ave., realized that the best way to write about aristocrats was through real estate.

"The whole idea that America has no caste system was the sub-theme of my eighth book, about Ralph Lauren," Gross told Business Insider. "And that notion, that there isn't one, is ridiculous."

"These buildings are a perfect way to tell a larger story," he said. "I wanted to write about the wealthiest, and most discerning people. ... Real estate is kind of the perfect vehicle to do that. Those buildings are natural. Only rich people can live there."

In the new documentary "Park Avenue: Money, Power, and the American Dream," a film directed by muckraking vet Alex Gibney and inspired by Gross' book on 740 Park Avenue, social disparity and the wealth gap are explored further. The documentary premieres Nov. 12 on PBS.

The film shows that those who live on the Manhattan's Park Avenue use their wealth and power to "manipulate the game" and "bend the rules" to help themselves get richer. In the documentary, experts talk about how presidential candidates would come to 740 Park Avenue to fundraise money for their campaigns.

Google MapsPark Avenue, Bronx

On the flip side, Park Avenue in the Bronx is the poorest congressional district in the country.

In the opening scenes of the documentary, the narrator evaluates the rare chance that someone growing up on the Bronx's Park Avenue would end up living on Manhattan's. Geographically, they are close (about a 10-minute drive), but in terms of lifestyle, the two couldn't be more different.

Of the 700,000 people who live in this district in the South Bronx, 40 percent live in poverty. The average person makes less than $40 a day, according to the documentary.

"From here the view has looked very different the past 30 years ... it's looked very different than the other Park Avenue," the narrator says. "They've seen their wages drop, and the prices of almost everything else go through the roof. They've lost their jobs because of bankers across the river. ... They're even worse off than they were a generation ago."

After a discussion of income disparity, the film delves into how the super-rich are spending their money: on real estate. In many cases, their Park Avenue condos are passed down along family lines, assuring that the wealth stays within the family.

via Google MapsGeographically, the Park Avenues are close. But socioeconomically, they are worlds apart.